Many types of touch panels are known and they include the resistive film type (analog resistive film type), the ultrasonic surface acoustic wave type, the infrared interruption type, the capacitive type, the electromagnetic induction type, and the image recognition type. Each type has its own advantages. Of these types, the present invention employs the resistive film type. Due to its simple structure, simple circuit connections, and low cost, the resistive film type is widely used for touch panels. In fact, the majority of currently commercialized touch panels are based on this type.
FIG. 6 shows a cross section showing the structure of a prior known resistive film type touch panel. In the figure, reference numeral 1 is the touch panel, 2 is a PET film, 3 is an upper ITO (Indium Tin Oxide) electrode, 4 is a glass substrate, 5 is a dot spacer, 6 is a lower ITO electrode, 7 is a double-sided adhesive tape, and 8 is a polarizing plate. Touch panel 1 comprises the glass substrate 4, the ITO electrode 6 formed on the glass substrate, the dot spacers 5 formed on the electrode, the PET sheet film 2 as an upper flexible substrate about 200 μm in thickness, and the polarizing plate 8 and ITO electrode 3 sandwiching the PET film therebetween. Spacers 5 are formed from an insulating material such as acrylate, urethane, or the like; the diameter of each dot spacer 5 is, for example, 50 μm, and the height is, for example, 5 μm to 6 μm. ITO electrodes 3 and 6 are transparent electrodes, and are formed over the entire area of the panel, i.e., over the lower surface of the PET film 2 and the upper surface of the glass substrate 4, respectively.
FIG. 6 shows the resistive film type panel in a condition in which the panel surface is not pressed with a finger or a pen tip. In this condition, no current flows between the ITO electrodes 3 and 6, because the electrodes are separated by the spacers 5. FIG. 7 is a schematic cross-sectional view of the resistive film type panel showing a condition in which the film surface is touched with a finger (or pen tip). In the figure, the pressing force causes the ITO electrodes 3 and 6 on the PET film 2 and the glass 4 to contact each other, and a current flows. When the film surface is touched, the resistive voltage dividing ratio is measured on each of the ITO electrodes 3 and 6 on the glass surface and the film surface, respectively, and the pressed position is thus calculated. This basic technology is described, for example, in Fujikura Technical Report, No. 102, April 2002, pp. 42-46, or in Japanese Unexamined Patent Publication No. H07-84705.
FIG. 8 shows the principle of how the coordinate point (X, Y) of a touch is calculated. FIG. 8(a) is a schematic diagram showing how the X coordinate is detected. Voltage Vcc is applied in the X direction on the upper film 2, and the resulting voltage is detected on the lower glass to calculate the X coordinate. Likewise, FIG. 8(b) is a schematic diagram showing how the Y coordinate is detected. Voltage Vcc is applied in the Y direction on the lower glass, and the resulting voltage is measured on the upper film to calculate the Y coordinate.
Currently commercialized touch panels are flat panels. However, in the fields of operation panels for game machines, automobiles and input/output devices for personal computers for which aesthetic appearance matters, there is a demand for a touch panel having a curved surface. As one example of a touch panel having a curved surface, there is disclosed, for example, in Japanese Unexamined Patent Publication No. 2005-182737 or in Japanese Utility Model Registration No. 3048333, a touch panel in which the same transparent resin film as that used for the upper substrate is also used for the lower substrate, thereby making the entire structure of the touch panel flexible so that the panel can, for example, be curved so as to conform with the surface on which it is mounted.